1. Field of the Invention
The present invention relates to variable speed transmissions and particularly to an improved transmission which employs a known type of continuously variable transmission in combination with a shifting gearbox to provide increased efficiency and power handling capabilities over a full speed range.
2. Description of the Prior Art
Presently available continuously variable transmissions have the ability to vary speed ratios in infinitesimal steps over a wide range of speeds to permit automotive engines to be operated at the most efficient speed for the load conditions and to be rapidly and smoothly shifted to a ratio that allows maximum engine power when required. However, use of the CVT has been severely limited by poor power handling capacity and relatively low efficiency of the CVT which negates most of the advantages in improving the operating efficiency of automotive engines. For high power ratings required for automotive applications, three types of CVTs are available. These are the hydrostatic, the flat belt, and the traction types.
The hydrostatic CVT, such as those manufactured by the Eaton Corporation, designated as Eaton Hydrostatic Transmissions Model 33 through Model 76, consist of a variable displacement hydraulic pump driving a fixed displacement hydraulic motor. These are commercially available in capacities up to 200 HP for use on heavy construction equipment and provide full load operating efficiencies of over 85%. The speed ratio is continuously variable from full forward to full reverse. The generally large size requirements and cost make this type unsuitable for automotive applications.
The flat belt type of CVT, such as one developed in Holland by van Doorne Transmissie, and utilized experimentally by Fiat, Renault and Ford automobiles, is similar to a conventional V-belt drive except that the belt is made wide so that the faces of the pulleys on which it rides can be moved together or apart. Moving the faces of one pulley together and the faces of the other pulley apart causes the belt to ride at a larger radius on the first pulley and at a smaller radius on the second pulley causing the speed ratio between the first and second pulleys to increase. This type of transmission is capable of operating at speed ratios in excess of 4:1, but with no reversal capability. It has been applied to low powered automobiles at ratings of up to 50 HP with efficiencies reported to approach 90%.
The traction type of transmission, such as described in U.S. Pat. No. 3,822,610 by Erban, typically consists of hardened steel rollers operating against a pair of toroidal discs. An extremely high contact force allows the rollers to transmit considerable power without slippage. Tilting the rollers changes the drive ratio between the discs. The device is capable of an efficiency of over 98% at full forward and 80% in full reverse with power ratings of several hundred horsepower being possible. However, the high cost of the required high strength materials limit its applicability. Thus, all of the known CVTs have operational, efficiency, cost and size disadvantages which limit use for general automotive applications.
One known example of an attempt to improve the use of a CVT by combining it with a gear mechanism is found in U.S. Pat. No. 3,527,119. This device, however, utilizes a unidirectional variable speed motor which can be selectively coupled to one of two plural paths to control the relative speeds. Each path can be stopped separately to achieve shifting without requiring braking but cannot be shifted under load. The variable speed drive is non-reversible and the device has only limited use for automotive applications.